In a large bowl, combine salt, pepper, paprika, garlic powder, and onion powder. Gradually whisk in the buttermilk to form a smooth mixture.
Submerge the shrimp in the buttermilk mixture, ensuring they are fully coated. If any shrimp remain exposed, add more buttermilk to cover. Cover the bowl with plastic wrap and refrigerate for 30 minutes.
Meanwhile, in another bowl, combine mayonnaise, sweet chili sauce, sriracha, and lemon juice. Taste and adjust the flavors using sweet chili sauce and sriracha based on your spice preference. Set aside.
After the marination period, place cornflour in a separate bowl. Remove each shrimp piece from the marinade, coat it in cornflour, and lay it on a tray. Continue this process until all shrimp are coated.
Heat oil in a deep frying pan or pot. Once hot, fry the shrimp in batches until they are golden brown and cooked through, approximately 2-3 minutes per batch. Transfer the fried shrimp to a wire rack or a plate lined with paper towels to drain excess oil.
Transfer crispy shrimp to a large bowl. Pour bang bang shrimp sauce over top. Gently toss to mix.
To serve, place a bed of shredded lettuce in a bowl, add the bang bang shrimp and garnish with scallions.